The photocatalytic degradation of Acid Orange 7 (AO7), a non-biodegradable ate-dye, has been investigated over TiO2 photocatalysts irradiated with a light source simulating solar light. The effect of operational parameters, i.e., dye concentration, photocatalyst content, pH of the solution and incident light energy on the degradation rate of aqueous solutions of AO7 has been examined. The effect of incorporating cations with valence higher (W6+) and lower (Ca2+) than the parent cation (Ti4+) in the TiO2 matrix has also been investigated. Results show that the employment of efficient photocatalysts and the selection of optimal operational parameters may lead to complete decolorization and to substantial decrease of the Chemical Oxygen Demand (COD) of the dye solutions.